As compared with traditional wire-based networks, optical fiber communication networks are capable of transmitting significantly more information at significantly higher speeds. Optical fibers, therefore, are being increasingly employed in communication networks.
To expand total transmission throughput, optical fiber network providers are attempting to place ever more optical fibers in ever-smaller spaces. Packing fibers into tight spaces, however, can cause undesirable attenuation. Indeed, there is an inherent trade-off between increased fiber density and signal attenuation.
Reduced-size cables, which are often desirable for certain installations (e.g., where space is limited), are requiring ever-smaller buffer tubes. As buffer tubes become increasingly small, however, excess fiber length (EFL) becomes a significant problem. As will be known by those having ordinary skill in the art, EFL can occur as a result of buffer-tube shrinkage during processing and thereafter as post-extrusion shrinkage (PES). This can lead to undesirable attenuation. In this regard, it is believed that smaller buffer tubes shrink more than larger buffer tubes under the same conditions.
Buffer-tube designs having somewhat higher optical fiber densities have been achieved for the deployment of standard single-mode fibers (SSMFs). For example, as many as twelve discrete, conventional optical fibers (e.g., SSMFs having a diameter of about 245-255 microns) have been deployed in loose buffer tubes with an outer diameter larger than 2.5 millimeters and an inner diameter larger than 1.6 millimeters. For SSMFs, however, as the buffer-tube filling coefficient approaches 0.3, attenuation becomes problematic, particularly at extreme temperatures (e.g., −40° C. or 70° C.). This is especially so with respect to mid-span storage performance, such as deployments in which SSMFs are positioned in pedestals, cabinets, or other optical-fiber enclosures. By way of example, loose-tube cables must be accessible multiple times along its installed length at various positions, typically at such optical-fiber enclosures.
By way of illustration, after installation in a duct, an optical-fiber cable typically experiences temperature cycles. These temperature cycles can lead to signal attenuation. Indeed, significant changes in temperature can lead to post-extrusion shrinkage and increases in excess fiber length (EFL), which may contribute to signal attenuation. Thus, a loose buffer tube that is less susceptible to post-extrusion shrinkage is more suitable for mid-span storage. It is generally accepted that cables containing buffer tubes having a lower buffer-tube filling coefficient are less susceptible to attenuation when subjected to temperature cycles and thus are more suitable for mid-span storage.
Reducing the wall thickness of a buffer tube while maintaining its outer diameter necessarily increases its inner diameter and thus the cross-sectional area available for deploying optical fibers. For many optical fiber applications, reducing buffer-tube wall thickness is unsatisfactory because such buffer tubes provide insufficient crush resistance (i.e., hoop strength). For many rigorous applications, buffer tubes must be capable of handling loads during installation and use in a way that satisfies customer expectations.
In addition, optical-fiber buffer tubes and fiber optic cables are susceptible to water intrusion. Water-blocking in buffer tubes and fiber optic cables typically has been accomplished by using petroleum-based filling compounds (e.g., grease or grease-like gels). By completely filling all of the free space inside a buffer tube that contains an optical fiber or optical-fiber bundle, the filling compound blocks the ingress of water into the fiber optic cable.
Moreover, being a thixotropic material, the filling compound also tends to mechanically couple the optical fibers to the buffer tube. Such mechanical coupling prevents the optical fibers from retracting inside the buffer tube as the buffer tube is processed during manufacturing, as the cable is installed or otherwise handled in the field, or as the cable is subjected to thermally induced dimensional changes from environmental exposure.
Although relatively effective for controlling buffer-tube and cable water-blocking, the petroleum-based filling compounds are inconvenient during cable access, cable repair, and optical-fiber splicing. The use of such filling compound requires cleaning the petroleum-based material from optical fibers prior to splicing (and sometimes from equipment and personnel, too). This can be messy and time consuming. Consequently, using conventional filling greases is often undesirable.
Various dry-cable designs have been developed to eliminate filling greases while providing some water-blocking and coupling functionality. In either loose-tube optical-fiber cables or ribbon cables, a totally dry design eliminates the filling compound from the enclosed buffer tubes. In a dry-cable design, for example, filling compound may be replaced by a water-blocking element, such as a tape or a yarn carrying a water-swellable material (e.g., water-swellable powder). Water-swellable powders are dry to the touch and, when bound to a carrier tape or yarn, can be readily removed during field operations (e.g., splicing).
A substantial problem with dry-cable designs is that the optical fibers can build up a charge of static electricity during processing. Static electricity may cause the optical fibers located within the buffer tube to repel one another. If the optical fibers are repelled from one another during buffer tube extrusion, they will be forced into and likely stick to the molten buffer tube wall. This sticking can result in elevated optical fiber attenuation. This static electricity may also persist until a buffer tube is opened for the purpose of accessing the optical fibers. A static charge is undesirable, because it may increase the difficulty of capturing optical fibers for splicing, connecting, or ribbonizing.
Another substantial problem with dry-cable designs is that after the formation of the extruded buffer tube around optical fibers, the optical fibers tend to stick to the inner surface of the solidified buffer tube. This stiction (i.e., the static friction between the optical fibers and the surrounding buffer tube) can result in increased and/or highly variable excess fiber length (EFL) during manufacturing. The stiction phenomenon can be the result of a static electricity, as well as simple contact and surface forces between an optical fiber and the surrounding buffer tube.
Proposed solutions to these respective problems of static electricity and buffer-tube sticking have been unsatisfactory. For instance, in gel-free buffer-tube designs that include a lubricant to reduce static and stiction, flow and leakage of the lubricant from the buffer tubes may occur during cable storage and installation.